Certain fluoropolymers bearing sulfonic acid functional groups are useful in the manufacture of polymer electrolyte membranes for electrolytic cells such as fuel cells. Examples include NAFION® (DuPont Chemical Company, Wilmington, Del.), which is a copolymer of tetrafluoroethylene (TFE) and a co-monomer according to the formula: FSO2—CF2—CF2—O—CF(CF3)—CF2—O—CF═CF2 that is sold in sulfonic acid form, i.e., with the SO2F end group hydrolyzed to SO3H. It is known to make such fluoropolymers by aqueous emulsion polymerization in the presence of ammonium perfluorooctanoate (APFO) as an emulsifier.
U.S. Pat. No. 3,635,926 purportedly discloses tough, stable copolymers of tetrafluoroethylene monomer and fluorovinyl ether monomers which can be produced by aqueous polymerization of the monomers by a process that requires that the reaction be carried out in the presence of a water-soluble initiator, an emulsifying agent and a gaseous chain transfer agent.
International Patent Application Pub. No. WO94/03503 purportedly discloses fluoroorganic polymeric material which comprises a polymeric chain having pendent groups including ion-exchange groups for use as an ion-exchange membrane in an electrochemical device. The application purportedly discloses preparation of the fluoroorganic polymeric material by aqueous emulsion polymerization using a emulsifying agent that is preferably a perfluoroalkanoate. ('503 at page 13).
Japanese Kokai Pat. App. No. 62-288617 purportedly discloses a method of producing a perfluorocarbon polymer having a sulfonic acid type functional group. The process is characterized in that emulsion copolymerization is carried out after emulsification of the liquid monomer is performed in an aqueous medium in the presence of a fluorine-containing emulsifier. The reference teaches the use of perfluorocarboxylic acid type emulsifiers at page 6.
U.S. Pat. No. 5,608,022 purportedly discloses a method for producing a perfluorocarbon copolymer containing functional groups which includes the steps of pre-emulsifying a perfluorocarbon monomer containing a functional group, adding at least one water-soluble organic chain transfer agent, and polymerizing with tetrafluoroethylene (TFE). The reference teaches the use of dispersing agents, e.g., at col. 6, lines 10–30.
U.S. Pat. No. 5,804,650 purportedly discloses preparation of a vinylidene fluoride (VdF) copolymer by emulsion-polymerizing VdF monomer with a reactive emulsifying agent. The resulting polymer latices are used suitably, for example, for paints. The reference teaches that the reactive emulsifying agent is present in an amount of 0.001 to 0.1% by mole in the combination of monomers ('650, col. 8, lines 58–60, and col. 17, lines 54–56). The reference also teaches that the reactive emulsifying agent is present in an amount of 0.0001 to 10% by weight relative to the weight of water in the emulsion ('650, col. 4, line 52, and col. 13, lines 10–17) and teaches against the addition of reactive emulsifying agent in any amount greater than 10% by weight relative to the weight of water in the emulsion. ('650, col. 22, lines 12–15) In Comparative Example 6, a tetrafluoroethylene (TFE) copolymerization is disclosed.